The present invention relates to an ultrasonic scanning apparatus having: a housing with a window through which an ultrasonic beam passes; a transducer fixed in the housing and having an ultrasonic transmitting/receiving surface for transmitting/receiving the ultrasonic beam; a reflecting member opposing the ultrasonic transmitting/receiving surface so as to form an angle therebetween, the reflecting member serving to reflect the beam from the ultrasonic transmitting/receiving surface so as to deliver the ultrasonic beam through the window; driving means for driving the reflecting member to change the propagation direction of the ultrasonic beam reflected by the reflecting member; and a motor having an output shaft coupled to the driving means.
The ultrasonic scanning apparatus of this type has been used in an ultrasonic diagnostic apparatus for performing tomogram so as to examine a part of the patient's body and form a tomogram thereof. In general, in order to obtain a tomogram of the heart or the like, the heart must be scanned with an ultrasonic beam from outside the body through a narrow space between adjacent ribs.
A typical example of a conventional ultrasonic scanning apparatus is illustrated in FIGS. 1 and 2. In this ultrasonic scanning apparatus, an ultrasonic beam is generated by a transducer. The ultrasonic beam is then projected from the transducer onto an ultrasonic reflecting mirror which is mechanically pivotable. The propagation direction of the ultrasonic beam is changed over time and the ultrasonic beam irradiates the portion of the patient's body to be examined, thereby performing mechanical sector scanning.
FIGS. 1 and 2 show a conventional ultrasonic scanning apparatus 10. A housing 12 of the apparatus 10 comprises a cylindrical portion 14 and a sector-shaped portion 16. The sector-shaped portion 16 extends in a direction substantially perpendicular to the central axis of the cylindrical portion 14. The sector-shaped portion 16 has an arc-shaped outer portion (i.e., an arcuated portion) 18. An arc-shaped window 20 is formed in the arcuated portion 18 and effectively transmits the ultrasonic beam therethrough. A partition wall 22 is formed in the cylindrical portion 14. The sector-shaped portion 16 extends downward from the part of the cylindrical portion 14 which is located to the left (FIG. 1) of the partition wall 22. A transducer 24 is mounted on the left end portion (FIG. 1) of the cylindrical portion 14 through a damper member 26 such that an ultrasonic transmitting/receiving surface 35 is perpendicular to the central axis of the cylindrical portion 14. A motor 28 and an angle detector 30 are mounted in the right-hand part (FIG. 1) of the cylindrical portion 14. The angle detector 30 is connected to the motor 28. An output shaft 32 of the motor 28 extends toward the transducer 24 through a bearing 34 and a seal member 36 so as to be coaxial with the central axis of the cylindrical portion 14. The bearing 34 and the seal member 36 are mounted in the partition wall 22. The seal member 36 prevents an ultrasonic propagation medium 38 (e.g., water containing a rustproof agent) in the sector-shaped portion 16 from leaking to the motor 28 through the seal member 36. A reflector (e.g., a mirror) 42 is rotatably disposed at the distal end of the output shaft 32 such that its reflecting surface 40 is inclined by a predetermined angle (45.degree. in FIG. 1) with respect to the ultrasonic transmitting/receiving surface 35.
When the transducer 24 and the motor 28 are powered, the ultrasonic beam from the transducer 24 is reflected by the rotating mirror 42, so that the ultrasonic beam is continuously rotated in one direction upon rotation of the mirror 42 in a plane substantially perpendicular to the central axis of the cylindrical portion 14. The mirror 42 is substantially perpendicular to the surface of drawing (FIG. 1) and its reflecting surface 40 is inclined to face the lower left corner. The ultrasonic beam reflected by the mirror 42 is reflected downward (FIG. 1) into the patient's body through the window 20 and a body surface 44. Therefore, the inside of the body is sector-scanned for an angular range determined by the size of the window 20. The ultrasonic beam transmitted into the body is reflected by the object to be examined and is received by the transducer 24 in a direction opposite to the forward propagation direction. The transducer 24 generates an electric signal which provides information about the inside of the body. This signal is supplied to an external electric device (not shown) for diagnosis of the inside of the body and reconstruction of a tomogram. A propagation path 46 has a width indicated by the alternate long and two dashed lines in FIG. 1. The ultrasonic beam is transmitted by the transducer 24 and is projected outside through the window 20 along the propagation path 46.
FIG. 2 is a sectional view of a structure taken along the line 2--2 in FIG. 1 when transducer 24, damper element 26 and housing 12 are partially eliminated from the apparatus 10 (FIG. 1) for illustrative convenience. Referring to FIG. 2, the ultrasonic beam can be projected through the window 20 disposed at the distal end of the sector-shaped portion 16 within a possible angular range W.sub.1. In fact, the ultrasonic beam is projected into the body between adjacent ribs 48 within an angular range W.sub.2.
The conventional apparatus 10 is very effective for ultrasonic diagnosis and tomogram reconstruction. However, strong demands for further improvements have arisen. First, since the cylindrical portion 14 extends perpendicularly to the sector-shaped portion 16 which is brought into tight contact with the body surface 44, an inclined angle of the distal end of the sector-shaped portion 16 with respect to the body surface 44 is greatly limited. Referring to FIG. 1, for example, when an operator such as a doctor wishes to project the ultrasonic beam from the upper right corner to the lower left corner and rotates the apparatus 10 clockwise, the cylindrical portion 14 extending to the right abuts against the body surface 44. A small angle with respect to the body surface 44 can not be obtained, resulting in inconvenience. When the doctor uses the apparatus 10 in a clinical examination, he searches for the object to be examined while watching the tomogram of the inside of the body. The doctor then moves and orients the sector-shaped portion 16 so as to precisely examine the desired object in different directions and angles. However, in the conventional apparatus 10, the inclined angle of the sector-shaped portion 16 with respect to the body surface 44 is limited as described above. In order to eliminate this drawback, it has been proposed that the length of the sector-shaped portion 16 be increased so as to increase the distance between the body surface 44 and the cylindrical portion 14. However, the size of the apparatus 10 is then increased and the scanning angular range is decreased, resulting in inconvenience. It should be noted that the decrease in the scanning angular range is apparent from FIG. 2. More particularly, when the sector-shaped portion 16 is increased in length in FIG. 2, the distance between the mirror 42 and the window 20 is increased. As a result, the possible angular range W.sub.1 is decreased. Along with this, since the distance between the mirror 42 and the ribs 48 is increased, the range W.sub.2 is also decreased.
Second, the distance between the mirror 42 and the window 20 is increased because of the presence of the sector-shaped portion 16. Therefore, the ranges W.sub.1 and W.sub.2 are restricted in practice.
Third, although the ultrasonic beam reflected by the mirror 42 is rotated through 360.degree. upon continuous rotation of the motor 28, ultrasonic beams reflected in any direction other than directions falling within the range W.sub.1 cannot contribute to sector scanning.
In order to eliminate these drawbacks, a method was proposed to periodically reverse the rotational direction of the motor 28 so as to swing the mirror 42 within the range W.sub.1 (FIG. 2). This leads to complex construction and control.